The present invention relates to helical tube heat exchangers and more particularly to such heat exchangers including two or more helically coiled tube bundles arranged in series with each other. More specifically, the present invention contemplates a tube heat exchanger of the type employed as a steam generator in conjunction with a nuclear reactor.
The development of nuclear power reactors entails the efficient and economical production of electrical power from thermal energy developed in the reactor. Within this field, it is necessary to operate the reactors at temperatures sufficiently high to enable the direct production of steam at temperatures and pressures suitable for high efficiency operation of steam turbines. For this reason, high temperature nuclear reactors have been developed which, when employed with a suitable steam turbine system, provide the capability of efficiently producing large quantities of electrical power.
The high temperature reactors are commonly enclosed in a pressure vessel through which a fluid coolant such as gaseous helium or carbon dioxide is circulated to withdraw thermal energy developed by the reactor. Steam for the operation of the turbines is obtained by the transfer of heat from the coolant to the fluid of a water/steam system. Such heat transfer is conventionally accomplished in a steam generator by withdrawing thermal energy from the reactor in the form of superheated steam.
In the design of steam generators for gas-cooled reactors, it is important to minimize the resistance to the flow of heat from gas to steam/water in the overall unit while at the same time employing design measures in individual sections of the steam generator to assure operation within prudent limits for temperature, material stress and other phenomena. It is also important, however, that there be as little restriction as possible to gas flow through the steam generator in order that work expended in transporting the gas be minimized.
A number of steam generators are commonly arranged within the same containment vessel as the nuclear reactor itself. Accordingly, it is important that each steam generator be of very compact size. However, to efficiently convert water to steam within the generator and to raise its temperature and pressure to satisfactory superheated conditions, it is necessary to provide a number of heat exchanger sections within each generator. These various sections are commonly formed as different tube bundles through which a fluid to be heated and vaporized, commonly water, flows in series while the primary coolant from the reactor is circulated about the tube bundles.
Typically, a steam generator may include a series of tube bundles with different tube bundles or different portions of the tube bundles acting as an economizer section for initially heating the water, an evaporator section wherein the water is converted to steam and any number of superheated sections, The superheater sections commonly include an initial superheater section, a plurality of intermediate superheater sections and a finishing superheater to heat the steam to desired temperature levels.
These various sections within a steam generator often have very different requirements which must be met by the design of the steam generator. For example, the relative amount of heat flux for each section of the steam generator must be carefully selected in order to achieve the desired effect within that section and within the steam generator as a whole.
Under different conditions, it is necessary to vary the point along the series arrangement of tube bundles where vaporization actually takes place. This of course affects the amount of heat transfer to be accomplished within each of the tube bundles and within different portions of each tube bundle. At the same time, it is desirable to design the various tube bundles so that heat transfer and pressure drop characteristics developed within any single bundle are not dictated by the geometry of another tube bundle within the generator.
Problems such as those outlined above, together with the need for maintaining a very compact configuration in the steam generator may create pressure drop penalties or increased complexity in the design of the steam generators. This is demonstrated in the following description which is directed toward a steam generator having the further requirement of a generally constant diameter along its length. This is not an essential limitation of the present invention however.
In meeting these complexities within the prior art, it has been common practice to allow the geometry of one tube bundle to dictate that of another, e.g., transverse tube pitch is the same in both bundles, or the number of tubes is varied in different tube bundles arranged in series to achieve the necessary operating characteristics for the steam generator. In the former case, substantial penalties in steam generator size, primary coolant pressure drop or tube wall temperature and thermal stress may result. In the latter case, extra headers or tubesheets must be provided at which the tubes in question can be terminated in order to permit a change in the number of tubes.
Accordingly, there has been found to remain a need for a design of helical tube heat exchangers wherein tube bundles arranged in series are simply and efficiently designed for maximum performance both of the individual tube bundles and the entire steam generator. This problem is especially critical within steam generators employed in conjunction with nuclear reactors in accordance with the preceding discussion. However, similar problems may arise in other applications for various vapor generators. Accordingly, the present invention is directed toward any vapor generator which involves design problems of the type outlined above.